Presently, charging schemes for charging Lithium Ion and Lithium polymer batteries in portable radios and cellular phones incorporate a ramped charge current approach. This ramped current approach is required because Lithium batteries typically contain a safety circuit that has an associated voltage cutoff value (V.sub.cutoff) associated to it. This cutoff value is set such that any current going into the battery cannot cause the cell to exceed this voltage value. If the voltage value is exceeded, the protection circuitry will open circuit the battery preventing further current from reaching the cell. This approach protects the cell from being overcharged and prevents catastrophic failure such as venting with flame. The ramped current approach is used to prevent a fully charged battery from overshooting the voltage cutoff (V.sub.cutOff) point with a current spike and accidentally causing an open circuit of the battery's overvoltage circuitry, thereby possibly damaging the battery and/or causing the radio to turn off.
The basic problem with the ramped current approach occurs when a user has a completely discharged battery (also referred to as a dead battery) and wants to transmit quickly. The ramping current is rather slow and causes an unwanted wait period to occur before the radio is capable of withstanding the transmit current. To illustrate the problem, the following example is described. Using a typical transmit current of 600 mA for a 3.6 Volt radio system, the ramp current will need to ramp up to a 600 mA level before enough charge current can be provided to the radio to maintain the transmit call. Further using a typical 4 mA per second charge ramp rate, the prior art ramp technique takes approximately 150 seconds (2 and 1/2 minutes) for the charge current to reach the point where the radio will be able to maintain the transmit load. Referring to FIG. 1, the prior art charge scheme wait period is depicted in graph 100. This wait period 102 can be very annoying to an end user who wants to quickly make a phone call after attaching a charger or power supply to his/her phone to charge a dead battery.
Accordingly, there is a need for an improved charging apparatus and technique for charging systems which minimize the unwanted wait period before transmit is allowed.